The present invention relates to a zoom lens system for use with a camera. More particularly, the present invention relates to a focusing method for a zoom lens system of a still camera.
Conventional focusing methods for zoom lens systems include a front focusing method and an inner focusing method, in which one of zoom lens units is moved for focusing. With these known focusing methods, however, there is a considerably large difference between aberration produced at a short object distance and aberration at infinity. Therefore, when the conventional focusing methods are used for close-up (macro photography), which is required for camera lenses these days, it is extremely difficult to obtain favorable image quality.
In the meantime, there have heretofore been focusing methods known as "floating system", such as a method wherein an aberration variation at a short object distance is corrected by moving two or more of zoom lens units independently of each other, and a method wherein one or a plurality of lenses in zoom lens units are moved independently to make such aberration variation correction. Examples of conventional floating systems include Japanese Patent Application Laid-Open (KOKAI) Nos. 1-204013 (1989), 2-10307 (1990), 4-338910 (1992), 4-406 (1992), 4-118613 (1992), and 2-73211 (1990).
These conventional floating systems suffer, however, from problems as stated below.
Japanese Patent Application Laid-Open (KOKAI) Nos. 1-204013 and 4-338910 are capable of suppressing the aberration variation at a short object distance to a certain extent but still insufficient to satisfactorily minimize the aberration variation.
In Japanese Patent Application Laid-Open (KOKAI) No. 2-10307, the variation of the image surface is favorably corrected, but the variation of spherical aberration at the tele end is unfavorably large because the first lens unit on the object side is used for floating.
In Japanese Patent Application Laid-Open (KOKAI) Nos. 4-406 and 4-118613, the first lens unit on the object side or the final lens unit on the image side is used for floating. Accordingly, the size of the lens to be moved is large, and hence the load on a mechanism for moving the lens increases, resulting in an increase in the overall size of the camera.
Japanese Patent Application Laid-Open (KOKAI) No. 2-73211 is similar in zoom lens type to the present invention, which will be described later. However, an aperture stop is disposed between the 2-nd and 3-rd lens units. Accordingly, when the aberration variation is suppressed by controlling the height of off-axis rays in a floating manner, as described later, the change of the height of off-axis rays caused by floating is small, so that the aberration variation cannot satisfactorily be suppressed. Particularly, the aberration correction effect at the wide end, where the depth of focus is shallow, is disadvantageously weak.